The advantages with lignin separation from black liquor is already described in WO 2006/031175 and WO2006/038863. These patents disclose the novel process LignoBoost™ that is now sold by Metso, and wherein WO 2006/031175 disclose the basic two stage acidic process and WO2006/038863 disclose an improvement of the process where sulphate or sulphate ions are added to the process.
An important aspect of the process is that the required charge of chemicals for the acidification may be high. If this is the case the cost of fresh chemicals is a large part of the operational cost and the commercial viability of the process is lower. These problems could be reduced, if the process is optimized for minimum requirement for charges of fresh chemicals, making the lignin product commercially sound.
Another problem with the process disclosed in WO 2006/031175 is that there may be a disposal problem with the strongly odorous H2S gases that are emitted from the reslurrying tank and bled out from the process, and it is suggested that these hydrogen sulfides could be added to the pulping process in order to increase sulfidity and possibly increase the yield in the pulping process. However, such rerouting of the strongly odorous H2S gases to another part of the pulp mill introduces risks for emissions of these gases during transport and storage. It is far better to use these gases at the location or process producing these gases.